Here's a list of the modal verbs in English:
Modals are different from normal verbs:
1: They don't use an 's' for the third person singular.
2: They make questions by inversion ('she can go' becomes 'can she go?')
3: They are followed directly by the infinitive of another verb (without 'to')
First, they can be used when we want to say how sure we are that something happened / is happening / will happen. We often call these 'modals of deduction' or 'speculation' or 'certainty' or 'probability'.
It's snowing, so it must be
very cold outside.
I don't know where John is. He could have missed
This bill can't be
right. £200 for two cups of coffee!